Cavity spot of carrot (Daucus carota)

Cavity spot disease of carrot (Daucus carota) has been one of the intractable problems for both growers and scientists. Carrots are rejected at grading with one or two visible lesions, and when disease incidence passes a relatively low threshold it becomes uneconomic to harvest crops. For the scientist, there has been considerable pressure to produce both information on the cause of the disease and a cure. Many putative causes have been advanced over the years, but these were almost always contradicted by subsequent work. The first solid indication of involvement of a pathogen was when three different fungicides with activity against Oomycete fungi all reduced disease. Very quickly the causal agents Pythium violae and Pythium sulcatum were isolated from cavity spot lesions and Koch's postulates satisfied. The species are not typical of the more common pythia, having slow growth at normal temperatures, which means that in the context of isolation work, plates may be overgrown by other species before they are seen. Metalaxyl fungicide was identified as the most effective in controlling cavity spot caused by P violae, but R sulcatum is naturally tolerant of the fungicide. Recently, metalaxyl has been shown to be subject to enhanced microbial degradation. This phenomenon has been associated with failure to control cavity spot. No other fungicide has been shown to be consistently effective in the field, and none has been registered for disease control. For the future, this means that control of cavity spot can not be based solely on fungicidal control. Other, complementary strategies are necessary for reducing disease. Calcium carbonate is known to have significant effects on cavity spot, probably by inducing a soil microflora inhibitory to filamentous fungi. Management of agronomic aspects such as irrigation, soil cultivation and the length of time for which crops are grown may all be used, while carrot cultivars with some field resistance may be beneficial. However, one of the most significant factors is disease avoidance by not selecting fields with high inoculum levels. One serology-based risk assessment test has been produced and commercialised, and molecular probes which could be the basis of more sensitive tests are available for both pathogens. The potential for disease reduction via a management strategy combining several key components is discussed.